1. Field of the Invention
The present invention relates to a method of controlling nucleation during the freezing step of a freeze drying cycle and, more particularly, to such a method that uses a pressure differential water vapor and CO2 ice fog and ice crystal distribution to trigger a spontaneous nucleation among all vials in a freeze drying apparatus and to minimize melting of ice crystals during flow from the condenser chamber to the product to be freeze dried.
2. Description of the Background Art
As described in my copending application Ser. No. 13/097,219, filed on Apr. 29, 2011, the new and improved controlled ice nucleation method utilizes the pressure differential between the seeding chamber (condensing chamber) and product chamber in a freeze dryer to instantly distribute the ice nucleation seeding crystals across the whole batch of product. Seeding ice crystals are originally generated inside a cold condensing chamber typically with a condensing surface below −80° C. Initially ice crystals exist in forms of frost on the condensing surface and frozen fog in suspension.
Once triggered by pressure differential distribution, frost breaks loose from the condensing surface mixing with frozen fog in suspension and rushes into the product chamber to trigger ice nucleation. During this travel between the seeding chamber and the product chamber, seeding flow has direct contact with surfaces at temperatures above 0° C. such as a vapor duct, isolation valve, baffle plate, product chamber wall, shelf stack parts and other surfaces. Depending on the complexity of the flow path, part of the seeding ice crystals melt before reaching the product surfaces.
This effect has great impact on ice nucleation efficiency in systems that have long or complex flow paths with obstacles at temperatures above 0° C. Some previous methods have compensated for the loss of seeding crystals by generating excessive amounts of seeding crystals and extended pre-cooling of the product chamber to reduce the temperature of obstacle surfaces. These compensation methods make the process less efficient in terms of time, material and energy.
Accordingly, a need has arisen for a new and improved method of reducing the melting of such ice crystals during their movement from the condensing chamber to the surfaces of the products in the freeze dryer. The method of the present invention meets this need.